1. Field of the Invention
This invention relates to a light scanning system, and more particularly to a light scanning system in which a deflected light bundle is imaged as a light spot on a surface to be scanned (will be referred to as xe2x80x9ca scan surfacexe2x80x9d, hereinbelow) by a catoptric system and the light spot is caused to scan the scan surface.
2. Description of the Related Art
There has been known a light scanning system in which a light bundle is once imaged as a line image on a deflecting surface of a deflecting means (e.g., a rotational polygonal mirror) and the line image of the light bundle is further focused on a scan surface as a light spot and the light spot is caused to scan the scan surface at a constant speed. The performance of an imaging optical system of such a light scanning system is generally evaluated mainly on the fxcex8 characteristic, bow of the scanning line, field curvature and/or the like, and various imaging optical systems have been designed for this purpose. For example, there has been disclosed, in Japanese Unexamined Patent Publication No. 11(2099)-153764, an optical system in which the line image on the deflecting surface is focused on the scan surface by a free curve mirror, and there has been disclosed, in Japanese Unexamined Patent Publication No. 8(1996)-220440, an optical system in which the line image is focused on the scan surface as a light spot by once reflecting the line image by a positive mirror and then reflecting the same by a negative mirror. Further, there has been disclosed, in Japanese Unexamined Patent Publication No. 8(1996)-211315, an optical system in which the line image is focused on the scan surface as a light spot by reflecting the line image by a pair of positive mirrors.
In a light scanning system for use in medical diagnosis, e.g., a laser printer for showing an image for medical diagnosis, there has been a demand that the light spot should scan the scan surface at a constant speed or that an error in fxcex8 characteristic (will be referred to as xe2x80x9can fxcex8 errorxe2x80x9d, hereinbelow) should be as small as possible, in order to suppress distortion of the reproduced image. However, in accordance with the catoptric system designing method based on the conventional (positive and negative) power distribution, it is difficult to design a scanning optical system which is sufficiently small in the fxcex8 error and can cause the light spot to scan the scan surface at a satisfactory constant speed.
In view of the foregoing observations and description, the primary object of the present invention is to provide a light scanning system which is sufficiently small in fxcex8 error.
In accordance with the present invention, there is provided a light scanning system for causing a light spot to scan a scan surface at a constant speed comprising a light source, a deflecting means which deflects a light bundle radiated from the light source, a line image imaging optical system which is disposed between the light source and the deflecting means and images a light bundle radiated from the light source on a deflecting surface of the deflecting means as a line image extending in a direction in which the light bundle is deflected, and a scanning/imaging optical system which is disposed between the deflecting means and the scan surface and images the light bundle deflected by the deflecting means on the scan surface as a light spot, wherein the improvement comprises that
the scanning/imaging optical system comprises a first aspheric mirror which is symmetric with respect to an axis of rotation and is disposed on the light inlet side of the scanning/imaging optical system and a second aspheric mirror which is anamorphic and is disposed on the light outlet side of the scanning/imaging optical system, the first aspheric mirror having a negative power in the direction in which a light bundle impinging upon the first aspheric mirror is deflected and the second aspheric mirror having a positive power in the direction in which a light bundle impinging upon the second aspheric mirror is deflected.
The line image imaging optical system may have a positive power in the direction in which the line image of the light bundle extends.
It is preferred that the scanning/imaging optical system satisfies the following formulae (1) and (2),
(|xcex1m1xc3x97efym1|xe2x88x92|xcex1m2xc3x97efym2|)/efy less than 35xe2x80x83xe2x80x83(1)
d0/efx greater than 0.4xe2x80x83xe2x80x83(2)
wherein xcex1m1 represents the inclination to Z-direction of the optical axis of the first aspheric mirror in YZ plane, efym1 represents the focal length of the first aspheric mirror, xcex1m2 represents the inclination to the Z-direction of the optical axis of the second aspheric mirror in Y-Z plane, efym2 represents the focal length of the second aspheric mirror on a cross-section along the Y-Z plane, efy represents the focal length of the scanning/imaging optical system on a cross-section along the Y-Z plane, efx represents the focal length of the scanning/imaging optical system on a plane normal to the Y-Z plane, and d0 represents the distance between the deflecting surface of the deflecting means on which the line image is imaged and the front principal point of the scanning/imaging optical system on a plane normal to the Y-Z plane, assuming that a direction in which the light spot is caused to scan the scan surface is the X-direction, a direction which is normal to the X-direction on the scan surface is Y-direction and a direction normal to both the X-direction and the Y-direction (i.e., a direction normal to the scan surface) is the Z-direction.
In the light scanning system of the present invention, since the scanning/imaging optical system comprises a first aspheric mirror which is symmetric with respect to an axis of rotation and is disposed on the light inlet side of the scanning/imaging optical system and a second aspheric mirror which is anamorphic and is disposed on the light outlet side of the scanning/imaging optical system, and the first aspheric mirror has a negative power in the direction in which a light bundle impinging upon the first aspheric mirror is deflected with the second aspheric mirror having a positive power in the direction in which a light bundle impinging upon the second aspheric mirror is deflected, that is, since the catoptric system is designed on the basis of a (positive and negative) power distribution different from that in the conventional catoptric system, a negative distortion is generated. As a result, the fxcex8 error can be smaller as compared with the conventional light scanning system. When the aforesaid formula (2) is satisfied, the fxcex8 error can be further smaller.
When the line image imaging optical system has a positive power in the direction in which the line image of the light bundle extends on the deflecting surface, the field curvature can be smaller.
Further, when the scanning/imaging optical system satisfies the aforesaid formula (1), bow of the scanning line can be suppressed.